[euroderf]

I'd try hanging long sheets of aluminum foil in too-large windows. I imagine someone in academia has studied this.

[sdeframond]

That works. A lot of people do that in their home when they have no other choice (renting the place, typically).

It'd be better to block the sun before it hits the window though, so that the window itself does not get hot and heat the inside.

Edit: people generally use emergency blankets, fyi. You can stick them to the glass with water.

[Timon3]

I've read a few times now that applying reflective foil to the inside using water can lead to broken windows - though I can't remember any linking to statistics or studies, so it could also be false or very rare.

[sdeframond]

I didn't know! Google agrees with you so it must be true.